1. Statement of the Technical Field
The inventive arrangements relate generally to methods and apparatus for providing increased design flexibility for RF circuits, and more particularly for optimization of dielectric circuit board materials for improved circuit performance.
2. Description of the Related Art
Transmission lines and radiating elements are commonly manufactured on specially designed substrate boards. For the purposes of these types of circuits, it is important to maintain careful control over impedance characteristics and coupling between lines. If the impedance of different parts of the circuit do not match, this can result in inefficient power transfer, unnecessary heating of components, and other problems. Coupling between lines can cause cross talk, electromagnetic interference (EMI), and adversely affect performance of the circuits
Printed transmission lines, passive RF devices, and radiating elements used in RF circuits are typically formed in one of three ways. One configuration known as microstrip, places the signal line on a board surface and provides a second conductive layer, commonly referred to as a ground plane. A second type of configuration known as buried microstrip is similar except that the signal line is covered with a dielectric substrate material. In a third configuration known as stripline, the signal line is sandwiched between two electrically conductive (ground) planes.
Via structures are commonly used in multilayered circuit boards to connect between different circuit board layers. Vias are typically oriented perpendicularly to the circuit board surface. Vias are sometimes strategically located on printed circuit boards between transmission lines to minimize RF coupling between the traces.
Further, conventional circuit board substrates are generally formed by processes such as casting or spray coating which generally result in uniform substrate physical properties, including the permittivity. Vias then are formed in the circuit board substrates after the casting or spraying step. Hence, manufacturing circuit boards with via structures is costly. Accordingly, via structure implementation has proven to be a limitation in designing circuits that are optimal in regards to both electrical characteristics and cost efficiency.
The present invention relates to a transmission line structure for reduced coupling of signals between circuit elements on a circuit board. The structure includes an RF transmission line disposed on a circuit board formed from a dielectric substrate material. The RF transmission line includes at least one elongated conductive metal trace having opposed elongated edge portions. The conductive metal trace and the substrate material are dimensioned for transverse electromagnetic wave propagation along the conductive metal trace at a selected frequency.
The structure also includes a pair of elongated substrate boundary regions. The substrate boundary regions are coextensive with at least a portion of the elongated conductive metal trace. Each of the boundary regions is positioned adjacent to a respective one of the opposing edge portions, defining an elongated substrate channel region. The substrate boundary regions define a transition from a first set of substrate characteristics to a second set of substrate characteristics wherein propagation of electromagnetic energy from the channel region to other portions of the circuit board is reduced by the presence of the boundary regions. For example, the permittivity of the substrate channel region can be smaller as compared to a permittivity of the dielectric substrate material surrounding the substrate channel region on an opposing side of the boundary region.
The transmission line structure can further include a pair of conductive metal traces disposed on the substrate channel region within the substrate boundary regions. The conductive metal traces are spaced apart and parallel to each other. An intermediate substrate region can be disposed between the pair of conductive metal traces. In one embodiment, a portion of each of the conductive traces is respectively disposed on an opposing face of the intermediate substrate region. The intermediate region has a permittivity different as compared to a permittivity of the substrate channel region. Optionally, the intermediate substrate region can extend through only a portion of a thickness of the circuit board on which the conductive metal traces are disposed.